Cj8 Vibration problem - help!

[Momojeep]

OK, so I'm trying to track down this annoying weird vibration problem. here's the deal:

-The original T-5 Transmission suddenly seized up on the freeway one day. I figured this must have been the problem. Replaced it with an AX-15. But the vibration still happens.
-It comes and goes. It usually happens when I drive over a bump, then goes away. It doesn't seem to matter if I'm in gear or not.
-It's a fast vibration like driving over a washboard. It rumbles and I can feel it in my seat. I do NOT feel it in the steering wheel/pedals/front end.
-Replaced the yoke/output shaft from the Dana 300 because a U-joint bearing cap was spinning a bit.
-Had rear driveline rebuilt and balanced, new U-joints.
-Got brand new wheels and tires.

-Still vibrates occasionally for short periods of time, then goes away, usually when I change speed.

-I noticed about .015" of play between the Ujoint retention tabs in the rear diff yoke. Could this be the problem, or part of it? (I did replace the Ujoint mount straps with new ones).
-I noticed the front output shaft from the Dana 300 has some play, and the front driveline is not straight (I can see it wobbling when in 4wd with the hubs disengaged). Could this be turning a bit when I'm driving even in 2wd causing a vibration? I used plastic twist ties to hold it still to see if the vibe goes still happens when I know the front driveline is not turning.
-I noticed my front wheel bearings have a little play.
- One of my engine mounts is not aligned well, and the rubber block is tweaked pretty good to make up for the misalignment, but seems to hold well.

Any thoughts?

 

[Hedgehog]

Sounds like you've got three more things to do. Start with your wheel bearings. Then motor mounts. Eventually you are going to have to address the front drive shaft.
................. Here's a thought. Check or have someone check the condition of your harmonic balancer. It seems like quite a few of those have gone bad lately.

 

[Torxhead]

Another possibility for that vibration over railroad tracks and washboard roads would be worn tie rod ends.

 

[Chilly]

Is your Jeep lifted, and by how much?

Have you made any adjustments to rear driveshaft angle by spring perch angled shims, or rewelding perches?

Is Transmission cross member factory, or raised up custom?

Did you slip your rear driveshaft apart, then reassemble splines with yokes out of phase?

 

[Bent]

AMC20 one piece axle conversions can cause vibrations after the bearings break in and loosen up a bit. Try jacking the rear wheels off the ground and jerk the top of the tire. If you think there might be too much end play, check to see if that side is shimmed and start removing the shims to tighten up the bearing preload.